1. Field of the Invention
The invention generally relates to liquid purification or separation. More specifically, the invention relates to membrane materials for separation of ethanol and water mixtures. The invention also relates generally to coating processes in which a permselective product is produced, specificially a thin, dense coating on a microporous substrate.
2. Description of the Prior Art
Ethanol is commonly produced by fermentation processes, wherein the ethanol product is found in a water mixture. The production of fuel-grade ethanol requires that the fermentation product be dried beyond the azeotrope. The usual drying process of distillation requires a significant amount of energy. Therefore, it is desirable to separate ethanol from fermentation beers by a more economical method, such as by membrane separation. In addition, significant preferential passage of ethanol at feed concentrations corresponding to fermentation beers can be a significant result because it may permit a fermentation process to operate at a low ethanol concentration while yielding a pervaporate sufficiently enriched for further processing by distillation or other means.
Selective membranes have been used in reverse osmosis processes, such as in the desalination of seawater and the separation of azeotropic mixtures of aromatic and aliphatic hydrocarbons or close boiling isomers. A principle disadvantage of reverse osmosis is that a high pressure is needed in excess of the prevailing osmotic pressure to drive the permeate through the membrane. Prevaporation avoids the limitation of osmotic pressure imposed on reverse osmosis processes by maintaining the permeate below its saturated vapor pressure. The heat of vaporization must be supplied to the permeating fraction in pervaporation, whereas during reverse osmosis there is no phase change and the heat of vaporization is not required. Thus, membranes used in the pervaporation process must meet more stringent membrane performance. To minimize energy input, membranes that pass water selectively would be of importance for solutions concentrated in ethanol, while membranes that pass ethanol selectively could remove ethanol directly from a fermentation bath. In either of these concentration regimes, osmotic pressures would hinder the competitive use of reverse osmosis.
The membrane separation of ethanol from water is difficult, and those membranes used for the separation of ethanol from either simple aqueous mixtures or from fermentation beers using reverse osmosis or pervaporation have been successful usually only in achieving a permeate that is enriched in water. A small number of exceptions to this result have been noted in published literature, as follows. It is reported in Heisler, E. G., A. S. Hunter, J. Siciliano, R. H. Treadway, Science, Vol. 124, p. 77, 1956, that adding benzoic acid to the feed yielded a slight enrichment of ethanol in the permeate when used with a cellophane membrane. Eustache, H., and G. Histi, J. Membr. Sci., Vol. 8, p. 105, 1981, report the use of pervaporation with a membrane of polydimethylsiloxane to yield a permeate enriched in ethanol. However, the latter measurements used feeds of only very low ethanol concentrations (ca. 0.1-1.0%). Finally, Hoover, K. C., and S. T. Hwang, J. Membr. Sci., Vol. 10, p. 253, 1982, report the use of a silicone rubber membrane in a pervaporation column with good separation factors at low ethanol concentrations; however, there was essentially no separation at very high ethanol concentrations. Thus, the prior art has not produced a membrane that is well suited to the separation of ethanol from water over a wide range of concentrations.
A primary problem encountered in membrane technology used to separate ethanol from water mixtures remains the creation of a membrane material that optimizes the properties that permit high separation efficiency and permeability. Some of the factors that influence the permeation process is polymers include chemical composition, membrane homogeneity, and the imposed driving forces causing permeation. It remains unpredictable as to what membrane composition will best perform in these areas, as the mechanism or mechanisms of membrane separation remain somewhat controversial, although the general sorption-diffusion theory is supported by a growing body of evidence.
The efficiency of liquid permeation separations through polymer films depends primarily on whether there is an interaction, chemical or physical, between the solvent, solute, and polymer. The extent of the liquid-polymer interaction determines how swollen the polymer becomes. These interactions arise in general from polar-, steric-, nonpolar-, or ionic-character of each of the above three components in the membrane system. The overall result of their interactions determines whether solvent, solute, or neither is preferentially sorbed at the membrane-solution interface.
Further, it has been observed that the permselectivity of a polymeric material increases as the general level of flux rate decreases. This aspect of transport behaviour must be overcome for economic separation processes by appropriate changes in membrane geometry and by adjusting polymer composition, structure, and morphology to enhance transport behaviour of the chosen penetrant. Both the diffusion coefficient and solubility coefficient of a penetrant are quite sensitive to minor variations in polymer composition and structure, which provides a possibility to experimentally derive useful permselective membrane materials.
Changes in membrane geometry are of great importance, as flux is inversely dependent on film thickness, while permeability constants are independent of thickness. Consequently, a very thin film can be highly permselective with excellent overall fluxes of the desired penetrant species. However, the presence and damaging effects of pinholes or other defects increase with decreasing membrane thickness. In order to develop optimum thin film materials, it is therefore essential that the dependence of permeability on factors that control transport processes be understood.
The above noted factors, among others, demonstrate the difficulty faced in the development of a membrane having the combination of high selectivity and concurrent high flux of the premeating species. To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the membrane and method of manufacture of this invention may comprise the following.